They're still transitioning from pure gcc first to llvm-gcc then to llvm+clang, and blocks are available in all of them.

XCode 3.2 defaults to GCC 4.2 with LLVM optionally available and IIRC XCode 4 too (can't check as I downgraded for various reasons).

The blocks patchset against pure GCC exists, and the problem mostly lies in upstream GCC refusing patches whose copyright has not been assigned/transferred to the FSF (see https://lwn.net/Articles/405417/). The rationale is that a critical component such as GCC should not be at the mercy of multiple (possibly hundreds) conflicting parties and easing a possible relicense process to ensure its protection.

While I understand the rationale behind this, my opinion is that it feels bureaucratic to the point of hampering notable innovative contributions while favoring local forks which will inevitably end up dying, as maintaining a fork (whatever the patchset size) against the march of a behemoth like GCC is essentially hopeless.